A variety of devices utilize ultrahigh pressure tubing, fittings, and/or other components that must withstand extreme pressures. In waterjet systems, for example, liquid is often contained and directed through tubes at ultrahigh pressures (i.e., pressures in excess of 30,000 psi). At pressures of this magnitude, high stresses are developed within the tubes. Repeated application of these pressures can lead to metal fatigue and eventual mechanical failure of the tubes. For example, repeated pressurization cycles in a waterjet system can eventually initiate cracks at an inner wall of a tube along a plane of highest shear stress. This is known as stage I of the fatigue crack propagation process. The repeated pressurization cycles can subsequently propagate the cracks from the inner wall towards an outer wall of the tube perpendicular to the maximum applied alternating loads. This is known as stage II of fatigue crack propagation. As the pressurization cycles continue the crack can grow until the stress intensity at the crack tip reaches the fracture toughness of the material and the crack grows in an unstable fashion until through-wall failure takes place. This is known as stage III of the fatigue crack propagation process. Tubing that is not regularly inspected and replaced to avoid failure due to the repeated application of internal pressure loading can eventually suffer fatigue failure. However, frequent inspection and replacement of tubes in waterjet systems is expensive.
Currently, waterjet systems typically make use of stainless steel tubing having nominal outer diameters of ¼″, ⅜″, or 9/16″ and nominal inside diameters equal to approximately ⅓ of the outer diameter. These ultrahigh pressure (UHP) stainless steel tubes typically have long lengths, and thereby have high aspect ratios (ratio of length to inside diameter). By virtue of their material and dimensions, UHP tubes can undergo numerous pressurization cycles before succumbing to fatigue failure. Typical stainless steel UHP tubes used in waterjet systems, for example, have operational lifespans of approximately 30,000 pressurization cycles from atmospheric pressure to 60,000 psi.
In addition to selecting appropriate materials and dimensions for tubing used to contain and convey UHP liquid, certain manufacturing techniques can be used to increase the operational lifespan of the tubing. For example, beneficial residual stresses can be induced within sections of tubing to increase the resistance to fatigue failure. In one method of inducing beneficial residual stresses, tubes are subjected to a procedure known as hydraulic autofrettage. This process involves the containment of a fluid within a tube and pressurization of the fluid to a pressure sufficient to produce a desired plastic deformation within an inner portion of a wall of the tube. The plastic deformation produces a slight increase in an inside diameter of the tube, and creates residual stresses in the wall of the tube.
Although the entire wall thickness is under hydraulic autofrettage induced stresses, the innermost portion of the wall thickness is under the greatest amount of induced beneficial compressive stress. The residual compressive stresses produced by the plastic deformation include radial and tangential stresses, the latter of which can be particularly beneficial. The compressive stresses are at a maximum at the inside diameter of the tube, and by reducing or minimizing the maximum shear stress from pressure cycles the compressive stresses can delay crack initiation and slow the growth of cracks. The benefit of hydraulic autofrettage and the penetration depth of compressive stresses is dependent on the wall ratio of the tube (ratio of outside diameter to inside diameter), the tubing material strength, and the autofrettage pressure.
In addition to delayed stage I crack initiation, hydraulic autofrettage can also slow stage II fatigue crack propagation by reducing the maximum principal stresses incurred during pressure cycles. Accordingly, by reducing the detrimental effects of pressurization cycles, hydraulic autofrettage can increase the fatigue life of the tubing and increase the maximum allowable internal pressure a tube can withstand.
In the context of waterjet systems, hydraulic autofrettage can extend the mean operational lifespan of tubing by 40 to 50%. For example, rather than 30,000 pressurization cycles from atmospheric pressure to 60,000 psi, a waterjet system utilizing tubing that has undergone hydraulic autofrettage can often perform up to 45,000 of these pressurization cycles. Although this increase in the operational lifespan of tubing is beneficial, additional increases in the operational lifespan of tubing are desirable to provide additional decreases in maintenance and operational costs of waterjet systems.